This study demonstrates that epileptic seizures in unrestrained, spontaneously breathing animals produce very high elevations of osmolarity in blood plasma, for which release of lactic acid produced by muscular contractions appears to be mainly responsible. The high plasma osmolarity induces osmotic dehydration of the brain which lasts for several hours. Elevation of plasma osmolarity and dehydration of the brain are absent in animals in which muscular contractions were pharmacologically abolished and which were artifically ventilated. In both groups of animals, it was evident that the opening of the blood-brain barrier (BBB) to protein tracers induces an increase in water content, which eventually, may lead to development of edema. These studies indicate that epileptic seizures associated with violent muscular contractions result in dehydration of the brain which may last for several hours. Also, the studies indicate that opening of the blood-brain barrier to proteins during the epileptic seizures leads to edema in the affected brain regions.